Method and apparatus for encoding an image signal having an object by using the shape thereof

ABSTRACT

An image frame signal encoding method encodes a shape signal of an object in the digital image frame signal and decodes the encoded shape signal to provide a reconstructed shape signal and converts background pixel values in the digital image frame signal to extension values to thereby provide an extension image frame signal. And an image frame signal encoding method detects object blocks including one or more reconstructed object pixels therein and encodes a smallest process block having a dimension of L×L pixels and including all the reconstructed object pixels within each object block.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for encodingan image signal at a low bit-rate; and, more particularly, to a methodand an apparatus for encoding an image signal having an object byutilizing the shape of the object.

DESCRIPTION OF THE PRIOR ART

In various electronic applications such as high definition TV and videotelephone systems, a video signal may be transmitted in a digital form.When the video signal comprising a sequence of video "frames" isexpressed in a digital form, there occurs a substantial amount ofdigital data: for each line of a video frame is defined by a sequence ofdigital data elements referred to as "pixels". Since, however, theavailable frequency bandwidth of a conventional transmission channel islimited, in order to transmit the substantial amount of digital datathrough the fixed channel, a video signal encoding method is normallyused to compress the digital data.

One of such methods for encoding image signals for a low bit-rateencoding system employs the so-called object-oriented analysis-synthesiscoding technique (see Michael Hotter, "Object-OrientedAnalysis-Synthesis Coding Based On Moving Two-Dimensional Objects",Signal Processing: Image Communication, 2, 409-428(1990)).

According to the object-oriented analysis-synthesis coding technique, aninput image signal, which has moving objects, is divided into theobjects; and three sets of parameters, i.e., those for defining motion,contour and pixel data of each object, are processed through differentencoding channels.

In case of processing image data or pixels lying within an object, atransform coding technique for reducing spatial redundancies containedin the image data may be employed in the object-orientedanalysis-synthesis coding technique. One of the most frequently usedtransform coding techniques for image data compression is a DCT(discrete cosine transform) based block transformation coding technique,which converts a block of digital image data, for example, a block of8×8 pixels, into a set of transform coefficient data. This method isdescribed in, e.g., Chen and Pratt, "Scene Adaptive Coder", IEEETransactions on Communications, COM-32, No. 3, pp. 225-232 (March 1984).

In the DCT based block transformation coding technique, a background ornon-object region within a block is filled with, e.g., 0, an averagepixel value or a mirror image of an object region in the block, and thentransformation is carried out. FIG. 1A shows an object region and abackground region in the block. Referring to FIGS. 1B and 1C,conventional methods for filling the background region are illustratedfor a 1-dimensional case. Specifically, in FIG. 1B, the backgroundregion is filled with 0; and in FIG. 1C, the background region is filledwith an average pixel value in the object region.

Even though these methods have the advantage of being able to utilizetwo-dimensional DCT blocks used in conventional methods (such as JointPhotographic Experts Group: JPEG, Moving Pictures Experts Group: MPEG,H.261 etc.), it also introduces unnecessary or undesirable data in thebackground region of the image, and is, therefore, inefficient from adata compression point of view.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide amethod and an apparatus for effectively encoding an image frame signalhaving an object by utilizing the shape of the object, thereby improvingthe data compression efficiency thereof.

In accordance with one aspect of the present invention, there isprovided a method for use in an encoder of a digital image frame signalhaving an object, wherein the digital image frame signal is divided intoa plurality of equal-sized blocks of N×N pixels and includes objectpixels contained in the object and background pixels located outsidethereof, N being a positive integer, the method comprising the steps of:

(a) encoding a shape signal of the object in the digital image framesignal, the shape signal including data for the size, position andcontour of the object;

(b) decoding the encoded shape signal to provide a reconstructed shapesignal;

(c) converting the values of the background pixels in the digital imageframe signal to extension values obtained by using the values of theobject pixels, to thereby provide an extension image frame signal;

(d) forming a reconstructed object in the extension image frame signalbased on the reconstructed shape signal and detecting object blocksincluding one or more reconstructed object pixels therein, each of thereconstructed object pixels representing a pixel included in thereconstructed object;

(e) forming a smallest process block having a dimension of L×L pixelsand including all the reconstructed object pixels within each objectblock, L being a positive integer and

(f) encoding the smallest process block.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, in which:

FIGS. 1A to 1C represent different methods for filling a backgroundregion;

FIG. 2 provides a block diagram illustrating a digital image framesignal encoding apparatus in accordance with the present invention; and

FIGS. 3A to 3D show the conventional repetitive padding technique.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A digital image frame signal has a plurality of equal-sized blocks,wherein a typical size of a block ranges between 8×8 and 32×32 pixels.The digital image frame signal having an object includes object pixelscontained in the object and background pixels located outside theobject. The background pixels may be represented by pixels whose valuesare much larger or smaller than those in the range of the ordinary pixelvalues.

Referring to FIG. 2, there is shown a block diagram of an apparatus 20for encoding a digital image frame signal in accordance with the presentinvention. The encoding apparatus 20 comprises a first and a secondcoding channels 200 and 500, a padding device 300, a process blockgenerator 400 and a formatting circuit 600. The first coding channel 200includes a shape encoder 210 and a shape decoder 220 and the secondcoding channel 500 includes a transform coder 510, a quantizer 520 andan entropy coder 530.

As shown in FIG. 2, a shape signal including data for the size,position, and contour characterizing the shape of an object in thedigital image frame signal is provided from the shape detector (notshown) to the first coding channel 200 to be coded therein. At the shapeencoder 210, the shape signal is first encoded by using, e.g., a binaryarithmetic code of JPEG (Joint Photographic Experts Group) and then theencoded shape signal is supplied to the shape decoder 220 and theformatting circuit 600. The shape decoder 220 converts the encoded shapesignal back into a reconstructed shape signal. And the reconstructedshape signal is provided to the process block generator 400.

In the meantime, the padding device 300 performs a padding process forthe digital image frame signal inputted thereto by using a conventionalrepetitive padding technique to convert it to an extension image framesignal to thereby improve a data compression efficiency at the secondcoding channel 500. Referring to FIG. 3A, there is shown a digital imageframe, wherein the dotted region represents an object region. The valuesof the pixels on the contour of the object as shown in FIG. 3A areextended in the horizontal and the vertical direction to fill thebackground region as shown in FIGS. 3B and 3C, respectively, wherein thesequence performing the horizontal and the vertical extension may bedecided according to image characteristics. The horizontal extension maybe performed on a row-by-row basis, while the vertical one may beperformed on a column-by-column basis. The background regions leftunfilled after the horizontal and vertical extensions as shown in FIG.3D may be filled by using pixel values of the horizontal and verticalextended regions. The extension image frame signal obtained from thepadding block 300 is provided to the process block generator 400.

The process block generator 400 forms, first, a reconstructed object inthe extension image frame based on the reconstructed shape signalprovided from the shape decoder 220 and detects object blocks includingone or more reconstructed object pixels therein, wherein thereconstructed object pixel represents a pixel included in thereconstructed object. And the process block generator 400 forms asmallest process block including all the reconstructed object pixelswithin each object block and having a dimension of L×L pixels, L being apositive integer. The smallest process block is provided to thetransform coder 510 in the second coding channel 500.

The transform coder 510 transforms the image signal of the smallestprocess blocks in the spatial domain from the process block generator400 into a set of transform coefficients in the frequency domain byemploying, e.g., a discrete cosine transform (DCT) and provides the setof transform coefficients to the quantizer 520. At the quantizer 520,the set of transform coefficients is quantized by using a knownquantization method; and then the set of quantized transformcoefficients is fed to the entropy coder 530 for further processing.

The entropy coder 530 encodes the set of quantized transformcoefficients from the quantizer 520 by using, e.g., a combination ofrun-length and variable length coding techniques to generate an encodedimage frame signal. The encoded image frame signal from the entropycoder 530 is then provided to the formatting circuit 600.

The formatting circuit 600 formats the encoded shape signal from theshape encoder 210 in the first encoding channel 200 and the encodedimage frame signal from the entropy coder 530 in the second encodingchannel 500, to thereby provide a formatted digital image frame signalto a transmitter (not shown) for the transmission thereof.

While the present invention has been described with respect to theparticular embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

What is claimed is:
 1. A method for use in an encoder of a digital imageframe signal having an object, wherein the digital image frame signal isdivided into a plurality of equal-sized blocks of N×N pixels andincludes object pixels contained in the object, each object pixel havinga object pixel value and background pixels located outside thereof, eachbackground pixel having a background pixel value, N being a positiveinteger, the method comprising the steps of:encoding a shape signal ofthe object in the digital image frame signal to generate an shapesignal, the shape signal representing the size, position and contour ofthe object; decoding the encoded shape signal to provide a reconstructedshape signal; converting the background pixel values to extension valuesin accordance with the object pixel values to generate an extensionimage frame signal having all extension values; forming a reconstructedobject in the extension image frame signal in accordance with thereconstructed shape signal, to thereby detect reconstructed objectblocks including one or more reconstructed object pixels therein, eachof the reconstructed object pixels representing a pixel included in thereconstructed object; forming a reconstructed object in the extensionimage frame signal in accordance with the reconstructed shape signal, tothereby detect reconstructed object blocks including one or morereconstructed object pixels therein, each of the reconstructed objectpixels representing a pixel included in the reconstructed object;forming a smallest process block with all the reconstructed objectblocks, wherein the smallest process block has a dimension of L×Lpixels, L being a positive integer; and encoding the smallest processblock to generate encoded data; wherein the step of converting includesthe steps of: determining contour pixels around the object as a functionof the shape signal, the contour pixels representing the object pixelslocated along the contour of the object; and modifying the backgroundpixel values in accordance with the contour pixel values.
 2. The methodof claim 1, wherein the step of modifying includes the stepsof:extrapolating the contour pixel values toward the horizontal and thevertical directions to deduce horizontal and vertical extension valuesfrom the background pixel values on the horizontal and the verticaldirections of the contour pixels; and filling the remaining backgroundpixels in accordance with the horizontal and vertical extension values.3. The method of claim 2, wherein the step of extrapolating extrapolatesthe horizontal and the vertical extension values on a row-by-row basisand a column-by-column basis, respectively.
 4. Apparatus for encoding adigital image frame signal having an object, wherein the digital imageframe signal is divided into a plurality of equal-sized blocks of N×Npixels and includes object pixels contained in the object, each objectpixel having an object pixel value and background pixels located outsidethereof, each background pixel having a background pixel value, N beinga positive integer, comprising:an encoder for encoding a shape signal ofthe object in the digital image frame signal, the shape signal havingdata for the size, position and contour of the object; a decoder fordecoding the encoded shape signal to provide a reconstructed shapesignal; a converter for converting the background pixel values toextension values using the object pixel values to generate an extensionimage frame signal; a device for forming a reconstructed object in theextension image frame signal from the reconstructed shape signal anddetecting object blocks including one or more reconstructed objectpixels therein, each of the reconstructed object pixels representing apixel included in the reconstructed object; a device for forming asmallest process block having a dimension of L×L pixels and each objectblock having all the reconstructed object pixels within, L being apositive integer; and wherein the encoder is operable to encode thesmallest process block; wherein said converter includes: a device fordetermining contour pixels around the object as a function of the shapesignal, wherein the contour pixels represent the object pixels locatedalong the contour of the object; and a modifying device for modifyingthe background pixel values based on the contour pixel values.
 5. Theapparatus of claim 4, wherein said modifying device includes:anextrapolater for extrapolating the contour pixel values toward thehorizontal and the vertical directions, to thereby deduce horizontal andvertical extension values from the background pixel values on thehorizontal and the vertical directions of the contour pixels; and adevice for filling the remaining background pixels based on thehorizontal and vertical extension values.
 6. The apparatus of claim 5,wherein said extrapolater extrapolates the horizontal and the verticalextension values on a row-by-row basis and a column-by-column basis,respectively.